One of the most exciting times in Colorado geologic history was the Pennsylvanian period when continental collisions formed the Ancestral Rocky Mountains. Europe and Africa collided with North America forming the Appalachian and Ouachita Mountains, and North America tried to break in two through southern Oklahoma. As the continent was being squeezed and ripped apart, the Uncompahgre highland was uplifted in central and western Colorado, the Front Range and Apishapa uplifts to the east, and the Central Colorado Trough became a deep basin between the two highlands.
During the late Pennsylvanian and Permian periods, massive amounts of sediment were shed off the Uncompahgre into the trough. The coarsest sediments are called the Crestone conglomerate (see shredthegnar10's report). Boulders up to 10' in diameter rolled down the mountainsides and were deposited in a pile of sediment that reached more than 9000' in thickness, a massive amount of bouldery rock. It was an environment similar to that of Red Rocks and Garden of the Gods, but must have been much steeper with more dramatic landslides and mudflows. There is very little fine grained sediment, but one notable band is much-beloved by readers on this website. Kit Carson Alley is a bench developed on a 4-6' thick mudstone which is amazingly continuous considering how little fine-grained sediment there is in the Crestones. There are still huge boulders in this mudstone, but its softness allows it to weather out and leave a path across Kit Carson Peak.
Ironically, the basin where those rocks were deposited has been inverted and now forms the spine of the Crestone Peaks area of the Sangre de Cristo Mountains. The cement that holds the boulders together makes the Crestone Conglomerate a stable and hard rock as many climbers appreciate.
The sediments form a 3000' high wall on these peaks, a massive amount of coarse-grained conglomerate to be deposited in narrow basin.

Crestone Peak, Crestone Needle, Kit Carson Peak, Challenger Peak, and Humboldt Peak all include rocks of the Pennsylvanian (323-299 million years ago) Minturn Formation and the Pennsylvanian-Permian (306-251 million years ago) Sangre de Cristo Formation.
The Sangre de Cristo Formation gradationally overlies the Minturn Formation (meaning that there is no missing time between them), and is defined by the redbeds near the basal part of the Sangre de Cristo Formation. The Minturn Formation consists largely of marine sediments (limestones, siltstones, shales), whereas the Sangre de Cristo Formation consists of primarily nonmarine sediments (arkosic conglomerates, sandstones, siltstones). The shift in the depositional environment resulting in these differences is interpreted as the result of a large-scale sea level regression that occurred during the Middle Pennsylvanian.
The Sangre de Cristo Formation consists of two members (members are kind of like subdivisions of a geologic formation): the Crestone Conglomerate and the Lower Member. The Crestone Conglomerate is defined by the presence of cobble(64-256mm diameter) and boulder (>256mm diameter) sized clasts.
Both the Sangre de Cristo Formation and the Minturn Formation formations were deposited in a sedimentary basin known as the Central Colorado Trough, which was created as a result of the uplift of the Ancestral Rocky Mountains, an indirect result of continental collisions involved in the formation of the supercontinent Pangea. Drainage systems developed to transport eroded material off of these mountains into the basin, and that material is what makes up these formations.
In the limestones of the Minturn Formation, scientists have identified numerous fossils, including fusulinids, brachiopods, crinoids, and bryozoans.
FUN FACT: The sediments of the Sangre de Cristo Formation were deposited during the same time period as the rocks of the Maroon Formation, which make up the Maroon Bells and Pyramid Peak.